40 year old wires

Sounds like a ground problem. Check the main body ground in the engine comparment. Pull the rear bulbs and check the grounds there as well.

Do you know how to do a "voltage drop" test? Find a KNOW GOOD ground (in this case you may need to go all the way to the battery) and put one lead of your volt meter on it. Put the other lead on the ground at the bulb (actually touch the cooper body if possible) with the bulb TURNED ON. If you have more than 0.2 volts showing you have a poor ground.

HTH

Brian Gordon